Avian renal proximal tubule epithelium urate secretion is mediated by Mrp4.
Birds are uricotelic and, like humans, maintain high plasma urate concentrations (approximately 300 microM). The majority of their urate waste, as in humans, is eliminated by renal proximal tubular secretion; however, the mechanism of urate transport across the brush-border membrane of the intact proximal tubule epithelium during secretion is uncertain. The dominance of secretory urate transport in the bird provides a convenient model for examining this process. The present study shows that short hairpin RNA interference (shRNAi) effectively knocked down gene expression of multidrug resistance protein 4 (Mrp4; 25% of control) in primary monolayer cultures of isolated chicken proximal tubule epithelial cells (cPTCs). Control and Mrp4-shRNAi-treated cPTCs were mounted in Ussing chambers and unidirectional transepithelial fluxes of urate were measured. To detect nonspecific effects, transepithelial electrical resistance (TER) and sodium-dependent glucose transport (Iglu) were monitored throughout experiments. Knocking down Mrp4 expression resulted in a reduction of transepithelial urate secretion to 35% of control with no effects on TER or Iglu. Although electrical gradient-driven urate transport in isolated brush-border membrane vesicles was confirmed, potassium-induced depolarization of the plasma membrane in intact cPTCs failed to inhibit active transepithelial urate secretion. However, electrical gradient-dependent vesicular urate transport was inhibited by the MRP4 inhibitor MK-571 also known to inhibit active transepithelial urate transport by cPTCs. Based on these data, direct measure of active transepithelial urate secretion in functional avian proximal tubule epithelium indicates that Mrp4 is the dominant apical membrane exit pathway from cell to lumen.
Bataille
Amy M
AM
University of Connecticut, Dept. of Physiology and Neurobiology, 75 North Eagleville Rd., Torrey Life Sciences Bldg. Rm. 67, Storrs, CT 06269, USA.
Goldmeyer
James
J
Renfro
J Larry
JL
D000818:Animals; D030161:Avian Proteins / Q000235:genetics / Q000378:metabolism*; D001692:Biological Transport; D002478:Cells, Cultured; D002645:Chickens; D017097:Electric Impedance; D004847:Epithelial Cells / Q000187:drug effects / Q000378:metabolism*; D005947:Glucose / Q000378:metabolism; D007687:Kidney Tubules, Proximal / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D008564:Membrane Potentials; D008871:Microvilli / Q000378:metabolism; D027425:ATP-Binding Cassette, Sub-Family C Proteins / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism*; D011422:Propionates / Q000494:pharmacology; D011804:Quinolines / Q000494:pharmacology; D034622:RNA Interference; D034741:RNA, Small Interfering / Q000378:metabolism; D051247:Sodium-Glucose Transport Proteins / Q000378:metabolism; D014527:Uric Acid / Q000378:metabolism*
D030161:Avian Proteins; D005947:Glucose; D027425:ATP-Binding Cassette, Sub-Family C Proteins; D011422:Propionates; D011804:Quinolines; D034741:RNA, Small Interfering; D051247:Sodium-Glucose Transport Proteins; D014527:Uric Acid; C059141:verlukast